US7457231B2  Staggered pilot transmission for channel estimation and time tracking  Google Patents
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 US7457231B2 US7457231B2 US10/926,884 US92688404A US7457231B2 US 7457231 B2 US7457231 B2 US 7457231B2 US 92688404 A US92688404 A US 92688404A US 7457231 B2 US7457231 B2 US 7457231B2
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 H—ELECTRICITY
 H04—ELECTRIC COMMUNICATION TECHNIQUE
 H04L—TRANSMISSION OF DIGITAL INFORMATION, e.g. TELEGRAPHIC COMMUNICATION
 H04L25/00—Baseband systems
 H04L25/02—Details ; Arrangements for supplying electrical power along data transmission lines
 H04L25/0202—Channel estimation
 H04L25/022—Channel estimation of frequency response

 H—ELECTRICITY
 H04—ELECTRIC COMMUNICATION TECHNIQUE
 H04L—TRANSMISSION OF DIGITAL INFORMATION, e.g. TELEGRAPHIC COMMUNICATION
 H04L23/00—Apparatus or local circuits for systems other than those covered by groups H04L15/00  H04L21/00
 H04L23/02—Apparatus or local circuits for systems other than those covered by groups H04L15/00  H04L21/00 adapted for orthogonal signalling

 H—ELECTRICITY
 H04—ELECTRIC COMMUNICATION TECHNIQUE
 H04L—TRANSMISSION OF DIGITAL INFORMATION, e.g. TELEGRAPHIC COMMUNICATION
 H04L25/00—Baseband systems
 H04L25/02—Details ; Arrangements for supplying electrical power along data transmission lines
 H04L25/0202—Channel estimation
 H04L25/0212—Channel estimation of impulse response

 H—ELECTRICITY
 H04—ELECTRIC COMMUNICATION TECHNIQUE
 H04L—TRANSMISSION OF DIGITAL INFORMATION, e.g. TELEGRAPHIC COMMUNICATION
 H04L25/00—Baseband systems
 H04L25/02—Details ; Arrangements for supplying electrical power along data transmission lines
 H04L25/0202—Channel estimation
 H04L25/0224—Channel estimation using sounding signals
 H04L25/0228—Channel estimation using sounding signals with direct estimation from sounding signals

 H—ELECTRICITY
 H04—ELECTRIC COMMUNICATION TECHNIQUE
 H04L—TRANSMISSION OF DIGITAL INFORMATION, e.g. TELEGRAPHIC COMMUNICATION
 H04L27/00—Modulatedcarrier systems
 H04L27/26—Systems using multifrequency codes
 H04L27/2601—Multicarrier modulation systems
 H04L27/2647—Arrangements specific to the receiver
 H04L27/2655—Synchronisation arrangements
 H04L27/2662—Symbol synchronisation

 H—ELECTRICITY
 H04—ELECTRIC COMMUNICATION TECHNIQUE
 H04L—TRANSMISSION OF DIGITAL INFORMATION, e.g. TELEGRAPHIC COMMUNICATION
 H04L27/00—Modulatedcarrier systems
 H04L27/26—Systems using multifrequency codes
 H04L27/2601—Multicarrier modulation systems
 H04L27/2647—Arrangements specific to the receiver
 H04L27/2655—Synchronisation arrangements
 H04L27/2689—Link with other circuits, i.e. special connections between synchronisation arrangements and other circuits for achieving synchronisation
 H04L27/2695—Link with other circuits, i.e. special connections between synchronisation arrangements and other circuits for achieving synchronisation with channel estimation, e.g. determination of delay spread, derivative or peak tracking

 H—ELECTRICITY
 H04—ELECTRIC COMMUNICATION TECHNIQUE
 H04L—TRANSMISSION OF DIGITAL INFORMATION, e.g. TELEGRAPHIC COMMUNICATION
 H04L5/00—Arrangements affording multiple use of the transmission path
 H04L5/003—Arrangements for allocating subchannels of the transmission path
 H04L5/0048—Allocation of pilot signals, i.e. of signals known to the receiver
Abstract
Description
I. Field
The present invention relates generally to data communication, and more specifically to pilot transmission, channel estimation, and time tracking in a multicarrier communication system.
II. Background
Orthogonal frequency division multiplexing (OFDM) is a multicarrier modulation technique that effectively partitions the overall system bandwidth into multiple orthogonal frequency subbands. These subbands are also referred to as tones, subcarriers, bins, and frequency channels. With OFDM, each subband is associated with a respective subcarrier that may be modulated with data.
In an OFDM system, a transmitting entity processes data to obtain modulation symbols and further performs OFDM modulation on the modulation symbols to generate OFDM symbols. The transmitting entity then conditions and transmits the OFDM symbols via a communication channel. A receiving entity typically needs to obtain relatively accurate symbol timing in order to recover the data sent by the transmitting entity. The receiving entity often does not know the time at which each OFDM symbol is sent by the transmitting entity nor the propagation delay introduced by the communication channel. The receiving entity would then need to ascertain the timing of each OFDM symbol received via the communication channel in order to properly perform the complementary OFDM demodulation on the received OFDM symbol. The receiving entity also needs a good estimate of the response of the communication channel in order to perform data detection to obtain good estimates of the modulation symbols sent by the transmitting entity.
The transmitting entity expends system resources to support channel estimation and time tracking, and the receiving entity also consumes resources to perform these tasks. The resources used by the transmitting and receiving entities for channel estimation and time tracking represent overhead. Thus, it is desirable to minimize the amount of resources expended by both the transmitting and receiving entities for these tasks.
There is therefore a need in the art for techniques to efficiently support channel estimation and time tracking in an OFDM system.
Techniques for performing “staggered” pilot transmission, channel estimation, and time tracking in a multicarrier (e.g., OFDM) communication system are described herein. To allow a receiving entity to derive a longer channel estimate while limiting the amount of resources expended for pilot transmission, a transmitting entity may transmit a pilot on different groups of subbands in different time intervals (e.g., different symbol periods). N subbands in the system may be arranged into M nonoverlapping groups. Each group may include P=N/M subbands that are distributed across the N subbands. The transmitting entity may transmit the pilot on a different subband group in each time interval. The transmitting entity may select all M subband groups in M time intervals based on a pilot staggering pattern. Alternatively, the transmitting entity may use many or most of the M subband groups in different time intervals, so that a substantial number of all subbands usable for transmission in the system are used for pilot transmission in different time intervals. The substantial number of subbands may be, for example, all of the usable subbands, three quarter of the usable subbands, at least half of the usable subbands, or some other significant percentage of the usable subbands. The receiving entity may derive an initial impulse response estimate with P channel taps based on the pilot received on one subband group. The receiving entity may derive a longer impulse response estimate (with up to N channel taps) by filtering initial impulse response estimates obtained for a sufficient number of different subband groups, as described below.
The receiving entity may derive two longer impulse response estimates of two different lengths L_{1 }and L_{2}, which may be used for data detection/decoding and time tracking respectively, where L_{1}=S_{1}·P and L_{2}=S_{2}·P. Each longer impulse response estimate may be derived based on a different timedomain filter that filters S or more initial impulse response estimates obtained for S or more different subband groups, where S may be S_{1 }or S_{2}. For each longer impulse response estimate, the first P channel taps are for a “main channel”, and the remaining channel taps are for an “excess channel”. The coefficients for each timedomain filter may be selected based on various criteria. For example, the coefficients for the main channel may be selected to (1) cancel the excess channel, (2) suppress time variation in the main channel, (3) provide an unbiased estimate of the main channel, and so on. Details of the filtering are described below. Various aspects and embodiments of the invention are also described in further detail below.
The features and nature of the present invention will become more apparent from the detailed description set forth below when taken in conjunction with the drawings in which like reference characters identify correspondingly throughout and wherein:
The word “exemplary” is used herein to mean “serving as an example, instance, or illustration.” Any embodiment or design described herein as “exemplary” is not necessarily to be construed as preferred or advantageous over other embodiments or designs.
At transmitting entity 110, a transmit (TX) data and pilot processor 120 receives different types of data (e.g., traffic/packet data and overhead/control data) and processes (e.g., encodes, interleaves, and symbol maps) the data to generate data symbols. As used herein, a “data symbol” is a modulation symbol for data, a “pilot symbol” is a modulation symbol for pilot (which is data that is known a priori by both the transmitting and receiving entities), and a modulation symbol is a complex value for a point in a signal constellation for a modulation scheme (e.g., MPSK, MQAM, and so on). Processor 120 provides data and pilot symbols to an OFDM modulator 130.
OFDM modulator 130 multiplexes the data and pilot symbols onto the proper subbands and further performs OFDM modulation on the multiplexed symbols to generate OFDM symbols. For each symbol period, OFDM modulator 130 performs an Npoint inverse fast Fourier transform (IFFT) on N multiplexed symbols for N total subbands and obtains a “transformed” symbol that contains N timedomain samples. Each sample is a complex value to be transmitted in one sample period. OFDM modulator 130 then repeats a portion of each transformed symbol to form an OFDM symbol that contains N+C samples, where C is the number of samples being repeated. The repeated portion is often called a cyclic prefix and is used to combat intersymbol interference (ISI) caused by frequency selective fading. An OFDM symbol period (or simply, a symbol period) is the duration of one OFDM symbol and is equal to N+C sample periods. OFDM modulator 130 provides a stream of OFDM symbols to a transmitter unit (TMTR) 132. Transmitter unit 132 processes (e.g., converts to analog, amplifies, filters, and frequency upconverts) the OFDM symbol stream to generate a modulated signal, which is then transmitted from an antenna 134.
At receiving entity 150, the transmitted signal from transmitting entity 110 is received by an antenna 152 and provided to a receiver unit (RCVR) 154. Receiver unit 154 processes (e.g., filters, amplifies, frequency downconverts, and digitizes) the received signal and provides a stream of input samples. An OFDM demodulator (Demod) 160 performs OFDM demodulation on the input samples and provides received data and pilot symbols. A detector 170 performs data detection (e.g., equalization or matched filtering) on the received data symbols with a channel estimate from a channel estimator 172 and provides detected data symbols, which are estimates of the data symbols sent by transmitting entity 110. A receive (RX) data processor 180 processes (e.g., symbol demaps, deinterleaves, and decodes) the detected data symbols and provides decoded data. In general, the processing by OFDM demodulator 160 and RX data processor 180 is complementary to the processing by OFDM modulator 130 and TX data and pilot processor 120, respectively, at transmitting entity 110.
Channel estimator 172 derives impulse response estimates based on the received pilot symbols from OFDM demodulator 160 and further derives frequency response estimates used by detector 170. A synchronization unit 162 performs time tracking and determines symbol timing based on the impulse response estimates from channel estimator 172. OFDM demodulator 160 performs OFDM demodulation based on the symbol timing from unit 162.
Controllers 140 and 190 direct operation at transmitting entity 110 and receiving entity 150, respectively. Memory units 142 and 192 provide storage for program codes and data used by controllers 140 and 190, respectively.
Data and pilot may be transmitted in various manners in system 100. For example, data and pilot may be transmitted (1) simultaneously in the same symbol period using frequency division multiplexing (FDM), (2) sequentially in different symbol periods using time division multiplexing (TDM), or (3) using a combination of FDM and TDM. The N total subbands may also be used for data and pilot transmission in various manners. An exemplary data/pilot transmission scheme is described below.
The N total subbands may be arranged into M “interlaces” or disjoint subband groups. The M interlaces are disjoint or nonoverlapping in that each of the N total subbands belongs in only one interlace. Each interlace contains P subbands, where P·M=N. The M interlaces are given indices of m=1 . . . M, and the P subbands in each interlace are given indices of p=1 . . . P.
The P subbands for each interlace may be uniformly distributed across the N total subbands such that consecutive subbands in the interlace are spaced apart by M subbands. Each interlace m, for m=1 . . . M, may include P subbands with the following k indices:
(p−1)·M+m, for p=1 . . . P. Eq (1)
As shown in
In general, system 100 may utilize any OFDM structure with any number of total, usable, and guard subbands. Any number of interlaces may also be formed. Each interlace may contain any number of subbands and any one of the N total subbands. The interlaces may contain the same or different numbers of subbands. For clarity, the following description is for the interlace subband structure shown in
A communication channel between transmitting entity 110 and receiving entity 150 in OFDM system 100 may be characterized by either a timedomain channel impulse response or a corresponding frequencydomain channel frequency response. As used herein, and which is consistent with conventional terminology, a “channel impulse response” or “impulse response” is a timedomain response of the channel, and a “channel frequency response” or “frequency response” is a frequencydomain response of the channel. In a sampleddata system, the channel frequency response is the discrete Fourier transform (DFT) of the channel impulse response. This relationship may be expressed in matrix form, as follows:
H _{N×1} =W _{N×N} ·h _{N×1 }and h _{N×1} =W _{N×N} ^{H} ·H _{N×1}, Eq (2)
where h _{N×1 }is an N×1 vector for the impulse response of the communication channel;

 H _{N×1 }is an N×1 vector for the frequency response of the communication channel; W _{N×N }is an N×N Fourier matrix; and
 “^{H}” denotes a conjugate transpose.
The Fourier matrix W _{N×N }is defined such that the (l,n)th entry, W_{N} ^{l,n}, is given as:
where l is a row index and n is a column index.
The channel impulse response h _{N×1 }is composed of N channel taps, with each channel tap h_{l }being defined by a zero or nonzero complex gain value at a specific tap delay l. The channel frequency response H _{N×1 }is composed of N channel gains for the N total subbands, with each channel gain H_{k }being a complex gain value for a specific subband k.
If pilot symbols are transmitted on the P subbands in interlace m, then the received pilot symbols for this interlace may be expressed as:
Y _{m} =H _{m} ∘X _{m} +N _{m}, Eq (4)
where X _{m }is a P×1 vector with P pilot symbols sent on the P subbands in interlace m;

 Y _{m }is a P×1 vector with P received pilot symbols obtained by the receiving entity for the P subbands in interlace m;
 H _{m }is a P×1 vector for the actual channel frequency response for interlace m;
 N _{m }is a P×1 noise vector for the P subbands in interlace m; and
 “∘” denotes the Hadamard product, which is an elementwise product, where the ith element of Y _{m }is the product of the ith elements of X _{m }and H _{m}.
The vector H _{m }contains only P entries of the vector H _{N×1 }for the P subbands in interlace m. For simplicity, the noise N _{m }is assumed to be additive white Gaussian noise (AWGN) with zero mean and a variance of or σ^{2}.
An initial frequency response estimate may be obtained for interlace m, as follows:
Ĥ _{m} =Y _{m} /X _{m} =H _{m} +N _{m} /X _{m}, Eq (5)
where Y _{m}/X _{m}=[y_{m,1}/p_{m,1 }. . . y_{m,P}/p_{m,P}], and y_{m,i }and p_{m,i }are respectively the received and transmitted pilot symbols for the ith subband in interlace m; and

 Ĥ _{m }is a P×1 vector for the initial frequency response estimate for interlace m.
Ĥ _{m }contains P channel gain estimates for the P subbands in interlace m, which may be obtained based on P elementwise ratios of the received pilot symbols to the transmitted pilot symbols, as shown in equation (5). If interlace m contains unused subbands with no received pilot symbols, then extrapolation, interpolation, and/or some other technique may be used to estimate the channel gains for these unused subbands.
 Ĥ _{m }is a P×1 vector for the initial frequency response estimate for interlace m.
A Ptap impulse response estimate using interlace m may be obtained by performing a Ppoint IFFT on the initial frequency response estimate Ĥ _{m}, as follows:
ĥ _{m} =W _{m} ·W _{P×P} ^{H} ·Ĥ _{m}, Eq (6)
where ĥ _{m }is a P×1 vector for the impulse response estimate for interlace m;

 W _{P×P }is a P×P Fourier matrix with elements defined as shown in equation (3); and
 W _{m }is a P×P diagonal matrix containing W_{N} ^{−m,p }for the pth diagonal element, for
 p=1 . . . P, and zeros elsewhere, where
The channel component in the P elements of vector W _{P×P} ^{H}·Ĥ _{m }contains a phase ramp which may be expressed as: h_{m,p}=h_{p}·W_{N} ^{m,p}, for p=1 . . . P. The slope of the phase ramp is determined by the staggering phase m of interlace m. The phase ramp may be removed by multiplying each element of W _{P×P} ^{H}·Ĥ _{m }with W_{N} ^{−m,p }to obtain a corresponding element of ĥ _{m}. The P elements of ĥ _{m }may be expressed as: h_{p}=h_{m,p}·W_{N} ^{−m,p}, for p=1 . . . P.
ĥ _{m }contains P channel taps and is obtained based on Ĥ _{m}, which contains P channel gain estimates for the P subbands in interlace m. Since the actual channel impulse response h _{N×1 }is composed of N channel taps, the initial impulse response estimate ĥ _{m }is undersampled in the frequency domain by the P subbands in interlace m. This undersampling in the frequency domain causes aliasing of the channel impulse response h _{N×1 }in the time domain. The initial impulse response estimate ĥ _{m }may be expressed as:
where h _{N×1}=[h _{1} ^{T } h _{2} ^{T }. . . h _{M} ^{T}]^{T }is the fulllength actual channel impulse response;

 h _{s}, for s=1 . . . M, is a P×1 vector containing P channel taps in h _{N×1 }with tap indices of (s−1)·P+1 through s·P;
 n is a P×1 vector of noise for the initial impulse response estimate ĥ _{m};
“^{T}” denotes a transpose.
An “aliasing pattern” corresponding to staggering phase m may be defined as {W_{M} ^{s,m}}, for s=1 . . . M, and include the coefficients used for equation (7). The fulllength actual channel impulse response h _{N×1 }is composed of M segments. Each segment s contains P consecutive channel taps in h _{N×1 }and is represented by a vector h _{s}. Equation (7) indicates that the M segments alias and combine when undersampled in the frequency domain, and the combining coefficients are given by the aliasing pattern.
A longer impulse response estimate with more than P channel taps may be obtained by transmitting pilot symbols on multiple interlaces. One interlace may be used for pilot transmission in each symbol period, and different interlaces may be used for pilot transmission in different symbol periods. The use of multiple interlaces for pilot transmission allows the receiving entity to obtain a longer channel estimate, which may improve performance. By using all M interlaces for pilot transmission, it is possible to estimate the entire fulllength channel impulse response with N channel taps.
The specific interlace to use for pilot transmission in each OFDM symbol period may be determined by a pilot staggering pattern. Various staggering patterns may be used for pilot transmission. In an embodiment, a staggering pattern may select one interlace for pilot transmission in each symbol period based on the following:
m _{t}=[(m _{t−1}−1+Δm) mod M]+1, with (Δm, M)=1, Eq (8)
where t is an index for symbol period;

 Δm is the difference between interlace indices for two consecutive symbol periods;
 m_{t }is the interlace to use for pilot transmission in symbol period t; and
 (x, y)=1 means that x and y are relatively prime (i.e., the greatest common divisor for both x and y is one).
The −1 and +1 in equation (8) account for an interlace index numbering scheme that starts with ‘1’ instead of ‘0’. The interlace used for the first symbol period is m_{1}, where m_{1}ε{1 . . . M}. Different “complete” staggering patterns may be formed with different values of Δm. A complete staggering pattern is one that selects all M interlaces for pilot transmission, e.g., in M symbol periods. As an example, with Δm=1, the M interlaces are selected in sequential order, and the staggering pattern may be expressed as {1, 2, 3, . . . , M}. For the case with M=8, values of 1, 3, 5, and 7 may be used for Δm to obtain different complete staggering patterns. Of these four values, 7 is equivalent to 1 (in terms of performance) since Δm=1 is an increment of one and Δm=7 is a decrement of one, and 5 is equivalent to 3 for the same reason.
In general, the pilot may be transmitted on any number of interlaces and on any one of the M interlaces in each symbol period. The particular interlace to use for pilot transmission in each symbol period may be selected based on any staggering pattern, three of which are shown in
A longer impulse response estimate {tilde over (h)} _{L×1}(t) with L channel taps, where P<L≦N, may be obtained by filtering multiple Ptap initial impulse response estimates obtained for multiple interlaces. This timedomain filtering may be performed, e.g., with a finite impulse response (FIR) filter, as follows:
where ĥ(t)=[ĥ_{1}(t) ĥ_{2}(t) . . . ĥ_{P}(t)]^{T }is an initial impulse response estimate obtained for symbol period t based on a pilot received on interlace m_{t};

 {tilde over (h)} _{s}(t)=[{tilde over (h)}_{s,1}(t) {tilde over (h)}_{s,2}(t) . . . {tilde over (h)}_{s,P}(t)]^{T }is a P×1 vector that is an estimate of the channel impulse response h _{s}(t) for segment s in symbol period t; α_{s,l}(i) is a coefficient for the ith filter tap used to derive the lth channel tap in segment s;
 N_{f }is the number of noncausal taps for the timedomain filter; and
 N_{b }is the number of causal taps for the timedomain filter.
The Ltap impulse response estimate {tilde over (h)} _{L×1}(t) is composed of S segments and may be given as: {tilde over (h)} _{L×1}(t)=[{tilde over (h)} _{1} ^{T}(t) {tilde over (h)} _{2} ^{T}(t) . . . {tilde over (h)} _{S} ^{T}(t)]^{T}, where S>1 and L=S·P. Each segment s, for s=1 . . . S, contains P channel taps that are included in the vector {tilde over (h)} _{s}(t). {tilde over (h)} _{s}(t) is an estimate of h _{s}(t), which is the actual channel impulse response for segment s.
Equation (9) indicates that the P channel taps for each segment s may be obtained by filtering N_{f}+N_{b }initial impulse response estimates ĥ(t+N_{f}) through ĥ(t−N_{b}+1), which may be obtained over N_{f}+N_{b }symbol periods for N_{f}+N_{b }different interlaces. The initial impulse response estimate ĥ(t) for the current symbol period t is aligned at filter tap i=0. Equation (9) also indicates that each channel tap {tilde over (h)}_{s,l}(t) in {tilde over (h)} _{L×1}(t) may be obtained by multiplying N_{f}+N_{b }channel taps ĥ_{l}(t−N_{b}+1) through ĥ_{l}(t+N_{f}) with N_{f}+N_{b }coefficients α_{s,l}(N_{b}−1) through α_{s,l}(−N_{f}), respectively, and combining the N_{f}+N_{b }resultant products.
In general, the coefficients for each channel tap {tilde over (h)}_{s,l}(t) of each segment s may be selected separately. Furthermore, N_{f }and N_{b }may be selected for each channel tap of each segment s. For simplicity, one set of N_{f}+N_{b }coefficients may be used for all P channel taps in each segment, and S sets of coefficients may be defined for the S segments of {tilde over (h)}_{L×1}(t). In this case, the coefficients {α_{s}(i)} for each segment s are not a function of channel tap index l.
The timedomain filtering may also be performed using other types of filter, such as an infinite impulse response (IIR) filter. The timedomain filtering may also be performed using a causal filter (with N_{f}=0 and N_{b}≧1), a noncausal filter (with N_{f}≧1), or a filter with both causal and noncausal taps. For clarity, the following description is for the timedomain filter shown in equation (9).
1. Channel Impulse Response Estimate of Length 2P
To obtain a longer impulse response estimate {tilde over (h)} _{2P×1}(t) with L=2P channel taps, the initial impulse response estimate ĥ(t) obtained in symbol period t for one interlace may be expressed as:
ĥ (t)= h _{1}(t)+ h _{2}(t)·W _{M} ^{m} ^{ t } +n(t), Eq (10)
where
Equation (10) is derived based on equation (7) and assumes that segments 3 through M contain channel taps with zero magnitude. The vector h _{1}(t) contains the first P channel taps in h _{N×1}(t) for the main channel. The vector h _{2}(t) contains the next P channel taps in h _{N×1}(t) for the excess channel.
The coefficients for the timedomain filter for the main channel estimate {tilde over (h)} _{1 }(t) may be selected based on various constraints such as:
 Cancel excess channel:
 Suppress time variation:
 Provide unbiased estimate:
and
 Minimize noise variance:
where m_{t−i }is the interlace used for pilot transmission in symbol period t−i, which corresponds to the ith filter tap. An unbiased estimate is one for which the mean of the estimate (over noise) is equal to the perfect channel value.
Equation (11b) cancels the linear component of the channel variation over the N_{f}+N_{b }symbol periods, which would be the dominant component at low speeds and/or small N_{f}+N_{b}. The first constraint in equation (11a) cancels the contribution from the excess channel h _{2}(t), so that {tilde over (h)} _{1}(t) contains mostly components from the main channel h _{1}(t). The second constraint in equation (11b) suppresses time variation in the main channel h _{1}(t) across the N_{f}+N_{b }symbol periods. The third constraint in equation (11c) provides an unbiased estimate of h _{1}(t), so that the expected magnitude of {tilde over (h)}_{1,l}(t) is equal to h_{1,l}(t). The fourth constraint in equation (11d) minimizes the noise variance in the main channel estimate {tilde over (h)} _{1}(t). The number of taps (N_{f}+N_{b}) for the timedomain filter determines (1) the number of degrees of freedom for selecting the coefficients and (2) the number of constraints that may be applied in selecting the coefficients.
The coefficients for the timedomain filter for the excess channel {tilde over (h)} _{2}(t) may be selected based on the various constraints such as:
 Cancel main channel:
 Suppress time variation of main channel:
 Suppress time variation of excess channel:
 Provide unbiased estimate:
The first constraint in equation (12a) cancels the contribution from the main channel h _{1}(t), so that {tilde over (h)} _{2 }(t) contains mostly components from the excess channel h _{2}(t). The second constraint in equation (12b) suppresses time variation in the main channel h _{1}(t). The third constraint in equation (12c) provides an unbiased estimate of h _{2}(t).
As a specific example, a 3tap timedomain filter may be used to derive the 2P channel taps in {tilde over (h)} _{2P×1}(t) based on ĥ(t−1), ĥ(t), and ĥ(t+1) for three symbol periods. The 3tap timefilter may be designed as follows. Using equation (10), the lth channel tap in symbol periods t−1, t, and t+1, prior to the timedomain filtering, may be expressed as:
ĥ _{l}(t−1)=h _{1,l}(t−1)+h _{2,l}(t−1)·W _{M} ^{m} ^{ t−1 } +n _{l}(t−1),
ĥ _{l}(t)=h _{1,l}(t)+h _{2,l}(t)·W _{M} ^{m} ^{ t } +n _{l}(t), for l=1 . . . P,
ĥ _{l}(t+1)=h _{1,l}(t+1)+h _{2,l}(t+1)·W _{M} ^{m} ^{ t+1 } +n _{l}(t+1), Eq (13)
where ĥ_{l}(t), h_{t,l}(t), h_{2,l}(t), and n_{l}(t) are the lth element of ĥ(t),

 h _{1}(t), h _{2}(t), and n(t), respectively; and
 m_{i−1}, m_{t}, and m_{t+1 }are the interlaces used for pilot transmission in symbol periods t−1, t, and t+1, respectively.
For the 3tap timedomain filter for staggering pattern 410 shown in

 Cancel excess channel: α_{1}(−1)·e^{−j3π/4}+α_{1}(0)+α_{1}(1)·e^{j3π/4}=0,
 Suppress time variation: α_{1}(−1)−α_{1}(1)=0, and
 Provide unbiased estimate: α_{1}(−1)+α_{1}(0)+α_{1}(1)=1.
The first equation above (to cancel the excess channel) is from equation (11a) and has the form: α_{1}(−1)·W_{8} ^{m} ^{ t } ^{+3}+α_{1}(0)·W_{8} ^{m} ^{ t }+α_{1}(1)·W_{8} ^{m} ^{ t } ^{−3}=0, which may be simplified as: α_{1}(−1)·W_{8} ^{3}+α_{1}(0)+α_{1}(1)·W_{8} ^{−3}=0, where W_{8} ^{3}=e^{−j3π/4 }and W_{8} ^{−3}=e^{+j3π/4}.
The solution to the above set of equations for the main channel is given as:
Equation (14) indicates that the coefficients for the main channel estimate {tilde over (h)} _{1}(t) are independent of symbol period t. This set of coefficients suppresses time variation in the main channel h _{1}(t) but does not suppress timevariation in the excess channel h _{2}(t). Timevariation error is, proportional to the energy of the channel taps, which is typically small for the excess channel and significant only when the transmitting and/or receiving entity is moving at high speeds. Thus, not suppressing time variation in the excess channel h _{2}(t) may only marginally degrade performance, if at all.
For the 3tap timedomain filter for staggering pattern 410 shown in

 Cancel main channel: α_{2}(−1)+α_{2}(0)+α_{2}(1)=0,
 Suppress time variation: α_{2}(−1)−α_{2}(1)=0, and
 Provide unbiased estimate: α_{2}(−1)·e^{−j3π/4}+α_{2}(0)+α_{2}(1)·e^{j3π/4}=e^{j2π(m} ^{ t } ^{−1)/8}.
The third equation above (to provide an unbiased estimate) is from equation (12c) and has the form: α_{2}(−1)·W_{8} ^{m} ^{ t } ^{+3}+α_{2}(0)·W_{8} ^{m} ^{ t }+α_{2}(1)·W_{8} ^{m} ^{ t } ^{−3}=1, which may be simplified as: α_{1}(−1)·W_{8} ^{3}+α_{1}(0)+α_{1}(1)·W_{8} ^{−3}=W_{8} ^{m} ^{ t }, where m_{t}ε{1 . . . M}.
The solution to the above set of equations for the excess channel is given by:
Equation (15) indicates that the coefficients for the excess channel are dependent on the staggering phase m_{t }of interlace m_{t }used for pilot transmission in symbol period t.
2. Channel Impulse Response Estimate of Length 3P
To obtain a longer impulse response estimate {tilde over (h)} _{3P×1}(t) with L=3P channel taps, the initial impulse response estimate ĥ(t) obtained in symbol period t for one interlace may be expressed as:
ĥ (t)= h _{1}(t)+ h _{2}(t)·W _{M} ^{m} ^{ t } +h _{3}(t)·W _{M} ^{2m} ^{ t } +n(t), Eq (16)
Equation (16) is derived based on equation (7) and assumes that segments 4 through M contain channel taps with zero magnitude. The vectors h _{1}(t), h _{2}(t), and h _{3}(t) contain P channel taps for the first, second, and third segments, respectively, of h _{N×1}(t).
A 3tap timedomain filter may also be used to derive the 3P elements of {tilde over (h)} _{3P×1}(t) based on ĥ(t−1), ĥ(t), and ĥ(t+1) obtained in three symbol periods. Using equation (16), the lth channel tap in symbol periods t−1, t, and t+1, prior to the timedomain filtering, may be expressed in matrix form, as follows:
where
Equation (17) assumes that m_{t−1}=m_{t}−Δm and m_{t+1}=m_{t}+Δm. The 3tap timedomain filter does not have enough degrees of freedom to suppress time variation in h _{1}(t), h _{2}(t), or h _{3}(t). Thus, equation (17) further assumes that h _{1}(t), h _{2 }(t), and h _{3}(t) are constant over the three symbol periods t−1, t, and t+1.
A leastsquares estimate of h _{1}(t), h _{2}(t), and h _{3}(t) may be obtained as follows:
The 3tap timedomain filter for h _{1}(t), h _{2}(t), and h _{3}(t) may be expressed in matrix form, as follows:
For staggering pattern 410 shown in
The main channel estimate {tilde over (h)} _{1}(t) may be obtained by applying the coefficients α_{1}(1), α_{1}(0), and α_{1}(−1) to ĥ(t−1), ĥ(t), and ĥ(t+1), respectively. The excess channel estimate {tilde over (h)} _{2}(t) may be obtained by applying the coefficients α_{2}(1), α_{2}(0), and α_{2}(−1) to ĥ(t−1), ĥ(t), and ĥ(t+1), respectively. The excess channel estimate {tilde over (h)} _{3}(t) may be obtained by applying the coefficients α_{3}(1), α_{3}(0), and α_{3}(−1) to ĥ(t−1), ĥ(t), and ĥ(t+1), respectively.
The 3tap timedomain filter does not have sufficient degrees of freedom to apply many of the constraints shown in equation sets (11) and (12). The coefficients for this timedomain filter do not suppress time variation in the main channel h _{1}(t) or the excess channel h _{2}(t) and h _{3}(t). The various constraints described above may be applied by using a timedomain filter with more than three taps.
In general, a different set of coefficients {α_{s}(i)} may be derived for the timedomain filter for the impulse response estimate {tilde over (h)} _{s}(t) for each segment s. The coefficients for each segment s may be selected based on various constraints such as: canceling the other segments, suppressing estimation error due to time variation in the channel, providing an unbiased estimate of h _{s}(t), minimizing the noise variance in {tilde over (h)} _{s}(t), and so on. The number of taps for the timedomain filter determines the number of constraints that may be applied to the coefficients. Several exemplary 3tap timedomain filter designs have been described above. Other timedomain filters may also be designed based on the description above and are within the scope of the invention.
In general, a longer impulse response estimate with L channel taps may be obtained based on pilot symbols received on L different subbands in one or more symbol periods. The pilot may be transmitted on one interlace in each symbol period to limit the amount of overhead for the pilot. The pilot may be transmitted on different interlaces with staggered subbands in different symbol periods. This allows the receiving entity to obtain a longer impulse response estimate with more than P channel taps. A fulllength impulse response estimate with all N channel taps may be obtained if the pilot is transmitted on all M interlaces using a complete staggering pattern.
The receiving entity may derive a longer impulse response estimate {tilde over (h)} _{L×1}(t) of length L by filtering initial impulse response estimates ĥ of length P for a sufficient number of (S or more) different interlaces. If the pilot is transmitted on a different interlace in each symbol period, then the timedomain filtering may be performed over a sufficient number of (S or more) symbol periods to obtain ĥ _{L×1}(t). A progressively longer impulse response estimate may be obtained by filtering over more symbol periods. Timedomain filtering over fewer symbol periods provides better tracking of changes in the channel, is thus more robust to Doppler effects, and can provide an impulse response estimate with a shorter length. Timedomain filtering over more symbol periods increases error in {tilde over (h)} _{L×1}(t) due to changes in the channel over time, is less robust to Doppler effects, but can provide an impulse response estimate with a longer length.
A longer impulse response estimate contains excess channel taps. Since each channel tap contains the complex channel gain at that tap position as well as noise, a progressively longer impulse response estimate contains more information regarding the channel but also contains more noise. The noise from the excess channel taps may be viewed as noise enhancement resulting from extending the length of the channel estimate beyond P. If the excess channel energy is relatively small, or if the excess channel taps are not needed, then better performance may be achieved with a shorter impulse response estimate (e.g., {tilde over (h)} _{2P×1}(t)). If the excess channel energy is relatively large, or if the excess channel taps are pertinent, then a longer impulse response estimate (e.g., {tilde over (h)} _{3P×1}(t)) may provide better performance even with the noise enhancement. Channel estimates with different lengths may be derived and used for different purposes at the receiving entity.
3. Data Detection
For data detection, a longer impulse response estimate {tilde over (h)} _{2P×1}(t) with 2P channel taps may provide a good tradeoff between a longer channel estimate and additional noise from the excess channel. The longer channel estimate mitigates the deleterious aliasing effect shown in equation (7) due to undersampling the frequency domain, provides a more accurate estimate of the main channel h _{1}(t), and allows for estimation of the excess channel h _{2 }(t). The longer impulse response estimate {tilde over (h)} _{2P×1}(t) may be derived as described above.
Postprocessing may be performed on the L_{1 }channel taps in {tilde over (h)} _{L} _{ 1 } _{×1}(t) to further improve channel estimation performance (block 520). The postprocessing may include truncation, e.g., setting channel taps P+1 through L_{1 }for the excess channel estimate to zeros. The postprocessing may alternatively or additionally include thresholding, e.g., setting channel taps in the main and/or excess channel estimates having energy below a given threshold to zeros. The unprocessed or postprocessed longer impulse response estimate {tilde over (h)} _{L} _{ 1 } _{×1}(t) may then be extended to length N by zeropadding to obtain a vector {tilde over (h)} _{N×1}(t) of length N (also block 520). An Npoint FFT may then be performed on {tilde over (h)} _{N×1}(t) to obtain a frequency response estimate {tilde over (H)} _{N×1}(t) for all N subbands (block 522), as follows:
{tilde over (H)} _{N×1}(t)=W _{N×N} ·{tilde over (h)} _{N×1}(t). Eq (20)
Process 500 may be performed for each symbol period with pilot transmission.
{tilde over (H)} _{N×1}(t) contains N channel gains for the N total subbands and may be expressed as: {tilde over (H)} _{N×1}(t)=[{tilde over (H)} _{1} ^{T}(t) {tilde over (H)} _{2} ^{T}(t) . . . {tilde over (H)} _{M} ^{T}(t)]^{T}, where {tilde over (H)} _{m}(t) contains P channel gain estimates for P subbands in interlace m. The M frequency response estimates {tilde over (H)} _{m}(t) for the M interlaces may have different noise variances depending on the particular staggering pattern used for pilot transmission. In general, a staggering pattern that is more spread out (e.g., staggering pattern 410) may result in less noise variation across {tilde over (H)} _{m}(t) for the M interlaces than a staggering pattern that is more closely spaced (e.g., staggering pattern 400).
4. Time Tracking
The receiving entity performs time tracking to estimate and track symbol timing across different OFDM symbols. The symbol timing is used to capture a window of N input samples (often called an FET window) from among the N+C input samples for each received OFDM symbol. Accurate symbol timing is pertinent since performance of both channel estimation and data detection is affected by the placement of the FFT window. The timing of the received OFDM symbol for each symbol period may be estimated by deriving a longer impulse response estimate for that symbol period and detecting for the timing based on an appropriate criterion, e.g. maximizing the energy that falls within the cyclic prefix.
If pilot symbols are available on L different subbands and a timing reference is not available, then a longer impulse response estimate with L channel taps may be derived but only L/2 channel taps may be resolved without any ambiguity. This is because a negative timing error results in earlier channel taps aliasing and appearing at the end of the impulse response estimate. Thus, it is not possible to determine whether the channel taps at the end of the impulse response estimate are later channel taps (if the symbol timing is correct) or earlier channel taps that have aliased (if there is a negative timing error). A longer channel impulse response estimate with up to N channel taps may be obtained by filtering the initial impulse response estimates for M different interlaces. The resolvable length of the communication channel is increased by the use of the longer impulse response estimate.
The longer channel impulse response estimate has additional noise due to the excess channel taps, and greater error due to channel timevariations. However, time tracking is likely to be less sensitive to the additional noise since the goal of time tracking is to determine less detailed information such as the general location of the channel energy rather than the complex channel gains of each tap. Thus, the tradeoff between channel quality and length is consistent with the requirements for data detection and time tracking. Specifically, for time tracking, a longer impulse response estimate {tilde over (h)} _{3P×1}(t) with 3P channel taps may provide a good tradeoff between resolvable channel length and noise enhancement. For example, if P=512, then {tilde over (h)} _{3P×1}(t) contains 1536 channel taps, and up to 768 channel taps may be resolved without ambiguity. Once the symbol timing is known, the communication channel may be assumed to be 3P/2 taps long for data detection purpose. A 3P/2tap channel may be estimated by obtaining a longer impulse response estimate with 2P channel taps and truncating the last 256 channel taps.
In general, the same or different impulse response estimates may be used for data detection/decoding and time tracking. The use of the same impulse response estimate can reduce the amount of computation at the receiving entity. In this case, the channel length L and the timedomain filter for this impulse response estimate may be selected to provide good performance for both data detection and time tracking. Different impulse response estimates may also be used for data detection/decoding and time tracking in order to achieve better performance for both, and may be derived with two timedomain filters. The channel length and the timedomain filter coefficients for each impulse response estimate may be selected to provide good performance for data detection or time tracking.
Within channel estimator 172, a pilot detector 822 removes the modulation on the received pilot symbols and may perform extrapolation and/or interpolation to obtain the initial frequency response estimate Ĥ(t) composed of P channel gains for the P subbands in the interlace used for pilot transmission in the current symbol period t. An IFFT unit 824 performs a Ppoint IFFT on Ĥ(t) to obtain the modulated impulse response estimate ĥ _{m}(t) with P channel taps. A rotator 826 removes the phase ramp in the P elements of ĥ _{m}(t) and provides the initial impulse response estimate ĥ(t). A timedomain filter 830 filters the initial impulse response estimates ĥ(t) obtained for S_{1 }or more interlaces obtained in S_{1 }or more symbol periods and provides the longer impulse response estimate {tilde over (h)} _{L} _{ t } _{×1}(t) with L_{1 }channel taps. A postprocessor 832 performs postprocessing (e.g., truncation, thresholding, and so on) and zeropadding on {tilde over (h)} _{L} _{ t } _{×1}(t) and provides a vector {tilde over (h)} _{N×1}(t) with N channel taps. An FEY unit 834 performs an Npoint FFT on {tilde over (h)} _{N×1}(t) to obtain the frequency response estimate {tilde over (H)} _{N×1}(t) for the N total subbands. Channel estimator 172 may also derive a frequency response estimate {tilde over (H)} _{m}(t) for just one or more selected interlaces.
Within time tracking unit 162, a timedomain filter 840 filters the initial impulse response estimates ĥ(t) for S_{2 }or more interlaces obtained in S_{2 }or more symbol periods and provides the longer impulse response estimate {tilde over (h)} _{L} _{ 2 } _{×1}(t) with L_{2 }channel taps. A timing detector 842 determines the timing for the current received OFDM symbol, e.g., based on the energy of the channel taps in {tilde over (h)} _{L} _{ 2 } _{×1}(t). A time tracking loop 844 (which may be a loop filter) adjusts the symbol timing from the timing used for the current received OFDM symbol.
The pilot transmission, channel estimation, and time tracking techniques described herein may be implemented by various means. For example, these techniques may be implemented in hardware, software, or a combination thereof. For a hardware implementation, the processing units used for pilot transmission at the transmitting entity may be implemented within one or more application specific integrated circuits (ASICs), digital signal processors (DSPs), digital signal processing devices (DSPDs), programmable logic devices (PLDs), field programmable gate arrays (FPGAs), processors, controllers, microcontrollers, microprocessors, other electronic units designed to perform the functions described herein, or a combination thereof. The processing units used for channel estimation and time tracking at the receiving entity may also be implemented within one or more ASICs, DSPs, and so on.
For a software implementation, these techniques may be implemented with modules (e.g., procedures, functions, and so on) that perform the functions described herein. The software codes may be stored in a memory unit (e.g., memory unit 142 or 192 in
Headings are included herein for reference and to aid in locating certain sections. These headings are not intended to limit the scope of the concepts described therein under, and these concepts may have applicability in other sections throughout the entire specification.
The previous description of the disclosed embodiments is provided to enable any person skilled in the art to make or use the present invention. Various modifications to these embodiments will be readily apparent to those skilled in the art, and the generic principles defined herein may be applied to other embodiments without departing from the spirit or scope of the invention. Thus, the present invention is not intended to be limited to the embodiments shown herein but is to be accorded the widest scope consistent with the principles and novel features disclosed herein.
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JP2010093827A (en)  20100422 
KR20070007956A (en)  20070116 
CA2565527A1 (en)  20051124 
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JP2011066902A (en)  20110331 
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US7907593B2 (en)  20110315 
KR100878430B1 (en)  20090113 
JP5474728B2 (en)  20140416 
ES2322077T3 (en)  20090616 
JP5080546B2 (en)  20121121 
KR100925909B1 (en)  20091109 
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CA2699640A1 (en)  20051124 
TW200625881A (en)  20060716 
CA2699640C (en)  20110712 
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US20090129511A1 (en)  20090521 
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CN1981498B (en)  20100929 
US20050249181A1 (en)  20051110 
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